Resilience in Multilayer Networks

Transcription

1 Resiliene in Multilyer Networks Piet emeester n Mihel Gryseels, University of Gent-IM Ahim Autenrieth, Tehnil University of Munih rlo rinz, Itltel Lur stgn n Giulio Signorelli, SIRTI Roberto lemente n Muro Rver, SLT Anrzej Jjszzyk n riusz Jnukowiz, ITTI Kristof Vn oorselere, elgom Yohnosuke Hr, NTT ASTRAT T The integrtion of ifferent tehnologies suh s ATM, SH, n WM in multilyer trnsport networks rises mny questions regring the oorintion of the iniviul network lyers. speilly in the re of network survivbility, muh n be gine by better lignment of the heling tions tken by ifferent network lyers in se of outges. Survivbility issues enountere in multilyer environment inlue, mong others: how to ientify the originl filure use, how to ppoint for eh filure lyer responsible for its heling, how to let ifferent lyers interwork, n how to provie spre resoures in n effiient wy. he resiliene of teleommunition networks gets intense ttention of ll mjor plyers in the teleom worl. The option of wvelength-ivision multiplexing (WM), synhronous igitl hierrhy (SH), n synhronous trnsfer moe (ATM) tehnologies in trnsport networks hs resulte in n ever-inresing onentrtion of more trffi on fewer network elements. For instne, the filure of 40-wvelength WM system rrying 2.5 Gb/s SH signls n ffet up to 1,200,000 telephone lls. onsequently, outges use more burens thn ever before, prtiulrly to ustomers whose ommunitions re of vitl importne. Network survivbility, whih is the bility of network to reover trffi ffete by filures, hs gine ritil role in the esign of teleommunitions networks. A gret el of reserh hs been one on the esign of survivble network rhitetures n reovery shemes for the ifferent trnsport tehnologies (see [1, 2] for surveys). Sine the survivbility tehniques implemente toy minly ress iniviul tehnologies, the stuy of solutions onsiering n integrte system is prmount for further evelopment in this fiel. Some objetives for n integrte pproh to multilyer survivbility inlue: Avoiing ontention between the ifferent single-lyer reovery shemes Promoting oopertion n shring of spre pity Inresing the overll vilbility tht n be obtine for ertin investment buget eresing investment osts require to ensure ertin survivbility trget This rtile presents results from the uropen reserh projet PANL, 1 whih stuie the oorintion of reovery shemes present in ifferent lyers of trnsport network. Note tht this rtile oes not inlue the iruit-swithe lyers; see [3]. 1 Protetion Aross Network Lyers, Projets/PANL A FRAMWORK FOR MULTILAYR SURVIVAILITY STRATGIS Multilyer survivbility strts from the viewpoint tht multitehnology network onsists of stk of singlelyer networks. A lient/server funtionl reltionship hols between jent lyers of the multilyer stk. h lower network lyer provies trnsport funtionlity to higher lient lyers. In eh network lyer with the neessry flexibility for rerouting, single-lyer reovery sheme my be eploye. The presene of reovery mehnisms in multiple lyers of ATM-over-SH-over-WM networks hs severl rivers: Reovery shemes resiing in lower lyers (e.g., SH) often enble more effetive reovery from burensome filures like ble uts, while filures of higher-lyer equipment (e.g., n ATM swith) requires itionl resiliene in the higher lyer. The nee for ifferentition of servie relibility (e.g., ifferent gres for istint ustomers or prouts) my result in the instlltion of reovery shemes loser to the network lyer where trffi is tully injete in the network. The nturl evolution of teleommunitions networks my result in ing new survivble lyers to the existing ones (e.g., optil lyer survivbility). Pst reserh [1, 2, 4, 5] reognize the importne of proviing synergy between the heling tions of the ifferent network lyers. PANL worke out frmework for multilyer survivbility (Fig. 1), whih ompiles iretions tht n be tken by teleommunitions network proviers. The omponents of the frmework n be lssifie in options pertining to the single-lyer reovery systems n options pertining to multilyer intertions. The reovery options of iniviul tehnologies hve been stuie extensively in the sope of single-lyer survivbility [1, 2]. In the next setions we illustrte some of the multilyer options of our frmework in the ontext of ATM-over-SH trnsport networks. Nevertheless, mny of the esribe survivbility onepts re generi n pplible to other types of multilyer networks s well. MULTILAYR ROVRY APPROAHS To efine multilyer reovery pproh, the following question is rise: For eh filure, whih network lyers re responsible for its reovery? We will illustrte some /99/$ I I ommunitions Mgzine August 1999

2 pprohes with the exmple in Fig. 2 representing two-lyer network, where the SH lyer supports ntive SH pths (e.g., A--) n lient ATM lyer. For instne, ATM pth -b- (set up between ATM noes n ) utilizes ATM link pity supporte by SH pths A- n -. Reovery interworking strtegies Reovery t the Lowest Lyer A first reovery pproh, enote s reovery t the lowest lyer, is to reover the ffete servies in the lowest possible lyer. As suh, the survivbility is provie s lose s possible to the origin lyer of the filure. For exmple, ble ut A- (Fig. 2.) ffeting ATM pth - is resolve t the SH lyer. When ATM noe b fils, the ATM reovery sheme nees to be tivte to restore ffete trffi. euse of the orser swithing grnulrity of lower lyers, this reovery pproh is simpler in the number of ffete pths to reroute (ompre to reovery t the highest lyer, see below). ue to the oexistene of multiple shemes, some interworking funtionlity nees to be provie to orretly ssign n oorinte the reovery responsibilities of eh reovery sheme. The brek of ble A- will result in loss of the signl t SH noe, but t the sme time ATM noe will not reeive vli ATM ells nymore. After 4 ms, the ATM noe goes into n lrm stte ue to loss of ell elinetion. In the mentime, it my lso hve reeive lrms from the SH lyer, initing tht the fult use is in the SH lyer. Inee, in the event of n SH lyer filure, subsequent lrm messges will be psse through eh upper lyer fter they re generte [6]. However, it my hppen tht the lrm signl from the SH lyer oes not rrive fst enough (e.g., intermeite SH network elements ely the lrm signl propgtion, n the ATM lyer efet my be etete first). As result, Link <> pth-bse Multilyer reovery pproh Reovery t lowest lyer Reovery t highest lyer Reovery t multiple lyers entrlize <> istribute ontrol Single-lyer reovery options Preplnne <> ynmi route lultion Interworking strtegy Sequentil tivtion - bottom-up - top-own - ignosti Prllel tivtion Figure 1. The PANL frmework for multilyer survivbility. eite <> shre bkup filities ommon pool Protetion seletivity Tritionl pproh Multilyer spre pity esign the ATM lyer woul flsely onlue tht the filure hppene in its own lyer n my inorretly trigger the ATM lyer reovery mehnism. However, the network resoures re imensione to resolve SH lyer filures t the lowest lyer. This my rete sitution for ATM lyer resoure ompetition, leing to network ongestion n other unwnte behvior of the network. Reovery interworking is require to overome this problem. With reovery t the lowest lyer, every survivble lyer reserves some resoures for the rerouting of ffete pths. A single-lyer reovery mehnism nnot quire resoures in other lyers in rel time without brehing the inepenent opertion of ifferent lyers. This mens tht spre pity in one lyer nees to be supporte t ll times by lower lyers. In our exmple, ATM lyer pity provie by SH pths hs to be reserve to reroute virtul pths (VPs) roun filing ATM noe. In ition, some SH lyer pity hs to be reserve for SH lyer rerouting. The result is tht eh survivble lyer ereses the tul utiliztion of the pity, n the lowest lyer my thus be poorly use with this reovery pproh. This problem is further isusse lter. Restore ATM (protete in SH) SH onnetion SH bkup (protetion route) ATM bkup (protete in ATM) ATM lyer b ATM lyer b A A SH lyer SH lyer ) Reovery t the lowest lyer b) Reovery t the highest lyer Figure 2. Multilyer reovery pprohes. I ommunitions Mgzine August

3 Reovery t the Highest Lyer A seon multilyer reovery pproh, enote reovery t the highest lyer, onsists of reovering isrupte trffi in network lyer loser to the origin of the trffi. A higher-lyer reovery sheme n resolve filures hppening in lyers below. The exmple in Fig. 2b illustrtes tht n ffete SH pth rrying SH servies is reovere by SH network elements, while n ffete ATM pth (rrie over ifferent SH pths) is reovere in the ATM lyer. Proviing resiliene t the highest possible lyer hs the following vntges: A trnsport network often rries severl servie lsses with ifferent relibility requirements. It is generlly esier to provie multiple relibility gres when the survivbility shemes resie in higher lyers. Sine single reovery sheme suffies to protet the servie ginst filures ourring in every (lower) lyer, the implementtion omplexity of interworking between ifferent shemes n be voie. In this se, interworking merely involves tivting the responsible reovery sheme s fst s possible t the pproprite network lyer. On the other hn, the finer swithing grnulrity of the higher lyers omplites rerouting in the event of lower-lyer filures, beuse mny entities re then ffete t the sme time. For instne, 2.5 Gb/s SH ble brek my ffet mny thousns of VPs in the ATM lyer, eh of whih woul hve to be reroute iniviully in this pproh. This woul probbly slow own the reovery proess. Seon, when the server lyer pths supporting the higher network lyer re rther long, the reovery proess my involve reonfigurtion in network elements fr wy from the originl filure use. Most probbly, speil preutions will hve to be tken to pt the survivbility sheme of the higher lyer (to resolve the lower-lyer filure senrios). For exmple, with restortion t the ATM lyer it is envisge to ssemble VPs shring physil routes into VP groups s tehnique to reue the reovery efforts in se of physil filures. Reovery Interworking/sltion In multilyer networks, severl reovery shemes my be involve to resolve ertin filures. An interworking strtegy (lso lle esltion strtegy [5]) onsists of set of rules esribing when to strt n stop, n how to oorinte, the tivities of the ifferent reovery shemes. Two options were ientifie onerning the tivtion: strting two or more reovery shemes in prllel or strting them sequentilly. The prllel strtegy is fst n requires no ommunition or oorintion between shemes. The sequentil strtegy my result in longer overll reovery times thn prllel tivtion but is generlly esier to ontrol. A sequentil strtegy etermines the orer of tivtion of the shemes n oorintes the shemes, ensuring tht they re tivte t n pproprite moment. Two further questions nee to be nswere in se of sequentil interworking: t whih lyer to strt the reovery n when to eslte to the next lyer. The question of where to strt results in three pprohes: bottom-up, top-own, n in-between (ignosti). ottom-up interworking strts t the network lyer losest to the filure, ensuring very quik tivtion of the reovery mehnism. The upwr esltion tkes ple upon expirtion of holoff timer, reeption of reovery token [7, 8], or intervention of the mngement system. Top-own interworking lwys strts t the highest-lyer network n esltes ownwr. For exmple, fst protetion sheme in the highest lyer n be ombine with slower but more ost-effetive restortion sheme in lower lyer. The ltter sheme is use to restore the higher network lyer to its originl stte, enbling it to ope with ertin ouble filure senrios. The eision to strt t speifi lyer epens on ignosti interworking, mong others, on reeive lrms n gthere survivbility sttistis. A QUANTITATIV OMPARISON OF ROVRY APPROAHS In orer to ompre reovery pprohes in quntittive wy, the PANL projet rrie out reovery simultion n plnning se stuies on vriety of ATM-over-SH network n trffi moels. A omprison of multilyer reovery pprohes is epenent lso on the single-lyer reovery options. Sine our im is to illustrte the multilyer spet of the reovery pprohes, the following se stuies ssume similr mehnisms to be eploye in the iniviul lyers. We onentrte on pth protetion shemes (i.e., en-to-en reovery with eite bkup pths) in both lyers. Suh shemes re often use for the resiliene of premium-level trffi. The onsiere filure senrio inlue single ble uts, single SH noe filures, n single ATM noe filures. Reovery Performne omprison Figure 3 ompres the performne of reovery t the lowest lyer with holoff time interworking n the reovery t the highest lyer Men reovery rtio (%) Lowest lyer reovery 0.2 Highest lyer reovery Reovery time (s) ATM TOR SAV AL GN MIL2 SH only offie MIL PIS SH & ATM offie R PIA VI VR OL FIR ROM VN AN Figure 3. ) Performne omprison; b) 16-noe exmple network. 72 I ommunitions Mgzine August 1999

4 (1) (2) (3) (4) pproh for smple network with 16 SH noes, six ATM noes, n 36 links (Fig. 3b). The grphs show the verge reovery rtio of ATM lyer onnetions for ll omplete site filures (mesure t the ATM lyer). Suh performne vlues were obtine by simulting the reovery proeure for every site filure one fter the other using istribute, event-oriente multilyer simultion environment evelope within PANL [8]. The performne t of the simultion runs re ollete n the men reovery rtio is lulte from ll results. The most notieble hrteristi of the grphs is the twostep urve in reovery t the lowest lyer. The mjority of ffete s re reovere by the SH protetion, whih tkes up to 50 ms n rehes n verge reovery rtio of bout 78 perent for the given network. ATM pths trnsiting the file ATM n SH site must be reovere t the ATM lyer, sine the SH pth supporting them is terminte t the file noe. Suh multihop ATM pths re reovere by the ATM protetion sheme. This sheme is triggere fter expirtion of the holoff timer (100 ms). It nees 30 ms to setup the (preonfigure) trnsltion tble. Aitionlly, few milliseons signling time re neee for eh file pth ue to the proessing time of the reovery protool. With reovery t the lowest lyer, ll ATM servies were reovere fter pproximtely 250 ms. euse fst reovery of single SH onnetion reovers very lrge number of ATM pths jointly, the performne of the reovery t the lowest lyer interworking strtegy is generlly better thn tht of reovery t the highest lyer. Single ble uts, whih re the most probble filures in network, n be resolve ompletely in the SH lyer. The ATM lyer only protets multihop ATM pths ginst trnsit noe filures. In reovery t the highest lyer, ll ffete ATM pths re reovere in the ATM lyer. No holoff time is neee now, but the reovery is slower sine muh higher number of ATM pths hve to be reovere iniviully. The min influene on the ATM reovery rtio is not the number of noes, but the number of ffete ATM pths. In the exmple bove with low ATM emn, ll ffete servies were reovere fter 900 ms. uring simultions with more emns, this vlue inrese to 3.5 s. 2 More etils bout this plnning se stuy n be foun in [6]. ATM equipment ost ,553 SH equipment ost ,488 rrier ost ,031 ATM servie ost ,478 16,298 47,072 Tble 1. Investment osts for reovery t the highest lyer. (1) (2) (3) (4) ATM equipment ost SH equipment ost ,792 rrier ost ,301 ATM servie ost ,613 14,090 39,093 Tble 2. Investment osts for reovery t the lowest lyer. ost omprison 2 The reovery ost in the ifferent ses ws evlute by ssessing the mount of SH n ATM equipment n fiber systems require by eh reovery pproh. The mount of resoures ws etermine through network imensioning proess, using network plnning tools vilble within the onsortium [6]. The network ost origintes from ATM equipment (bkbone ATM X VP n its STM-1 interfe rs), SH equipment (X 4/4, STM-1 interfes, 2.5 Gb/s line systems), n rrier ost, whih inlues the ost of fiber n regenertors. Atul equipment osts obtine from survey of ifferent ATM n SH equipment mnuftures were use [6]. All osts in this rtile re expresse in normlize unit, orresponing to the ost of one eletril STM-1 port on n SH line system. In Tble 1 (n Tble 2), the investment osts relte to the ATM emns re liste for reovery t the highest lyer (n lowest lyer, respetively) for set of network n trffi senrios (1) (4). In ll ses, reovery t the highest lyer requires lrger investment buget (up to 20 perent more) thn reovery t the lowest lyer. There re severl explntions for this outome. First of ll, with reovery t the lowest lyer, only multihop VPs require itionl ATM lyer protetion (n subsequent ATM lyer spre pity). This results in lower ATM spre pity requirement. Seon, the pprohes iffer in the onstrints tht must be stisfie in fining working n bkup VP routes. With reovery t the highest lyer, the min n bkup routes of VP must be isjoint in both physil links n noes. With reovery t the lowest lyer, the only requirement is to hve no trnsit ATM noe in ommon. Hene, the reovery t the lowest lyer pproh les to looser VP routing onstrints n more room for optimiztion. Furthermore, the SH network lyer supports both working n spre ATM pity. With reovery t the highest lyer, no SH spre resoures hve to be provie for ATM emns, but the extr eployment of the ATM spre pity results in higher ouption of SH filities by the ATM lyer. In ft, the totl SH pity require for the ATM emns is bout the sme for both reovery pprohes. With reovery t the lowest lyer, prt of the spre pity for ATM servie resiliene is only put in the SH network lyer, n only hs n impt on the neee equipment (n instlltion osts) of one network lyer inste of two. onluing, it seems heper to resolve physil n SH filures using reovery t the SH lyer inste of eiting expensive ATM pity for this tsk. OMMON POOL SURVIVAILITY Sine trnsmission n swithing resoures re very expensive, optimiztion of spre resoures is very importnt for opertors (n iniretly for ustomers s well), beuse it results in ontinment of the osts n onsequently lso of the servie triffs. As expline before, multilyer survivbility implies proviing multiple spre pity pools, eh eite to prtiulr network lyer. Sine pity of eh survivble lyer is rrie by its server lyers, this results in reservtion of resoures in ll lyers below. With tritionl pity plnning, this results in poor utiliztion of the lowest lyer. Moreover, the server lyer potentilly lso protets the spre pity of the lient lyer, whih oes not lwys result in n equl inrese of servie vilbility. Suh reunnt protetion n be voie by supporting working n spre lient lyer pity through ifferent server pths, n treting them ifferently in the server lyer. For exmple, when protetion seletivity is vilble in the server lyer, pths rrying lient lyer spre pity n be left unprotete. Nevertheless, the server lyer then still eites some I ommunitions Mgzine August

5 A ATM lyer SH lyer b Figure 4. ommon pool survivbility n ATM filure senrio. ATM bkup (protete in ATM, extr trffi in SH) Working SH ring hnnel Protetion ring hnnel In summry, the spre pity require for ATM lyer resiliene is trete s unprotete preemptible trffi in the SH network lyer. As suh, the SH spre resoures re reuse for extr resiliene t the ATM lyer. A tritionl pproh eploys two spre pity pools, eh eite to speifi lyer (i.e., ATM or SH). The ifferene in relibility is tht with ommon pool it is no longer possible to provie resiliene ginst ommon filure of n ATM noe n ble ut. Sine suh filure senrios hppen rrely, the effet on servie vilbility is very smll. In ontrst, the svings in SH equipment hieve with ommon pool pproh n be quite substntil [10]. MULTIPART SURVIVAILITY IN SH-OVR-WM NTWORKS urrently, WM tehnology is minly use on point-topoint bsis to inrese the vilble trnsmission pity on existing fibers. The throughput growth possible with WM tehnology inreses the impt of filures, sine mny more onnetions fil simultneously. In the future, when optil AMs n ross-onnets beome vilble, there will be opportunities for full optil pths between offies fr from eh other. A ble ut then results in omplex filure senrio in the SH lyer. If reovery is left to SH, single ble ut my require mny swithing tions t ifferent lotions. Proviing survivbility t the WM lyers beomes inherently ttrtive s network throughput inreses [8, 11]. Optil reovery shemes reover multiple ffete SH links t one n entil less oorintion thn SH reovery. In the lst few yers, the reserh on WM reovery shemes hs therefore intensifie, n optil protetion swithing hs beome topi of interest in the stnriztion boies [9]. euse of the funtionl similrity of SH n WM networks, the survivble rhitetures in the WM lyer re likely to operte in similr wy to the stnrize SH rhitetures [9]. In relity, optil networking (n optil reovery shemes) will not be eploye t one in ll prts of the network. Suh WM islns my then be interonnete by pure SH trnsmission links (or links with point-to-point WM). The en-to-en survivbility of n SH onnetion thus nnot be ensure by WM protetion only. In ition, it is resoures to rrying the lient lyer spre pity. The utiliztion of the server lyer resoures n be further improve by shring spre pity ross network lyers s esribe in [10]. The bsi ie of ommon pool survivbility is to tret the spre pity of the lient lyer s extr trffi (i.e., rrie on unprotete preemptible pths) in the server lyer. The spre pity t the server lyer is plnne to protet the server lyer pths rrying the tul trffi. With ommon pool survivbility, the server lyer spre pity is reuse by higher-lyer reovery sheme. Little or no itionl server lyer resoures re thus require to support the lient spre pity, whih is now rrie in the reserve pity provisione for server lyer survivbility. The utiliztion of network resoures is better thn with ny other pproh. The onept of ommon pool survivbility is illustrte with simple network exmple shown in Figs. 4 n 5, onsiering network with meshe ATM lyer on top of SH biiretionl rings. Although the figure onsiers, for the ske of rebility, single SH ring, it hs lso been verifie for network onsisting of interonnete rings. In ft, it shoul be stresse tht the ommon pool survivbility onept is generi n n be pplie to other types of multilyer networks s well. A working ATM VP, -, route vi ATM noe b, is trnsporte vi the working SH pths A-- n -, s shown in Fig. 4. To protet this working VP ginst trnsit ATM noe brekowns, bkup ATM VP - route vi ATM noe is preestblishe. This bkup VP is trnsporte vi the SH pths A- n -. However, these SH pths re not route long working SH ring hnnels but s extr trffi long the protetion hnnels of the ring. If the ATM X b fils (Fig. 4), the working ATM VP - is reroute vi its pressigne bkup VP. This involves simple reservtion n reonfigurtion proess t the ATM network lyer only. After reovery ompletion, ATM VP - is trnsporte over SH pro- ATM lyer b tetion hnnels without requiring reonfigurtion of ny of the SH -rop multiplexers (AMs). On the other hn, when ble ut (Fig. 5) ours, the SH AMs ess the protetion hnnels to reroute the isrupte working SH pths. Hene, the SH pths supporting the ATM spre pity my be preempte in orer to free resoures for the ring protetion protool. This implies the temporl unvilbility of A some prt of the ATM spre pity until the filure is repire. There is no nee for tivtion of the ATM reovery, sine the SH pro- SH lyer tetion restores ll ffete VPs. Figure 5. ommon pool survivbility n SH filure senrio. ATM bkup route (unvilble) Working SH ring hnnel xtr trffi preempte 74 I ommunitions Mgzine August 1999

6 very iffiult to etet ertin filures (e.g., those using slow signl egrtion) t the WM lyer. Aitionl SH lyer survivbility will thus remin neessry in the network. se on the bove rtionle, severl protetion strtegies n be propose. The first option is to hve protete SH lyer over n unprotete WM lyer. For instne, in the urrent networks where WM is use for point-topoint systems in well-efine res, it is resonble to leve the protetion to SH, sine protetion mehnisms n mngement systems of the SH lyer re very mture. A seon option is to hve n unprotete SH over protete WM lyer. Whenever WM is not yet eploye in ll prts of the trnsmission network, this option uses WM protetion whenever possible n SH protetion in other prts of the network. As suh, this strtegy prtitions the network into ifferent survivble subnetworks, n filures re resolve within their subnetwork. Although suh prtitioning enbles survivl of filures hppening in ifferent subnetworks, it requires speil preutions t the gtewys, whih n be expensive to implement. Also, SH lyer filures within the WM islns re not overe. These ltter issues re not present with protete SH lyer supporte by protete WM lyer. However, if ombintion of the two protetion shemes is plnne without ny preutions, the spre pity of the SH lyer results in higher mount of wvelengths. Moreover, these wvelengths (prtly rrying spre SH pity) re protete in the WM lyer. This option my be too ostly sine WM lyer protetion of SH spre pity is not lwys useful. Reunnt protetion n be voie when working n spre SH pities re supporte by seprte wvelengths, using protetion seletivity t the WM lyer. Nevertheless, supporting SH spre pity by unprotete WM wvelengths still results in poorer utiliztion of the WM lyer thn in the other options. A ommon pool onfigurtion, where SH spre pity is supporte on preemptible wvelengths in the WM lyer, obtins optiml utiliztion. ONLUSIONS This rtile presents outomes of the PANL reserh projet, whih stuie survivbility in multilyer trnsport networks. The mjor hllenges for telo in multilyer environment re to etermine whih lyer(s) is responsible for eh filure (reovery pproh), to oorinte single-lyer reovery tions (esltion strtegy), n to pln multiple spre pity pools in n pproprite wy. In orer to provie overll servie resiliene in ost-effetive wy, servie proviers n etermine suitble strtegy bse on the PANL guielines (see box). PANL GUILINS Reovery t the highest lyer is the reommene pproh when: Multiple relibility gres nee to be provie with fine grnulrity Reovery interworking n not be implemente; (e.g., beuse of equipment limittions or when lient n server network lyer re owne by ifferent opertors) The survivbility shemes in the highest lyer re more mture thn in the lowest lyer Reovery t the lowest lyer is the reommene pproh when: The number of entities to reover hs to be limite/reue When the lowest lyer supports multiple lient lyers n it is pproprite to provie survivbility to ll servies in homogeneous wy The survivbility shemes in the lowest lyer re more mture thn in the highest lyer When it is iffiult to ensure the physil iversity of working n bkup pths in the higher lyers (e.g., when lient n server lyer re provie by ifferent opertors, the server network opertor my not wnt to iffuse informtion on his network topology) Reovery t the lowest lyer my require expliit oorintion between reovery shemes; the hol-off time bse interworking mehnism is the best ompromise between reovery performnes n implementtion omplexity. With reovery t the lowest lyer, multiple spre pity pools re present in the network for resiliene of higher lyer trffi. When higher lyer spre pity requirements re reltively lrge, the tul utiliztion of the lower lyers n beome poor. In tht se, using unprotete or preemptible server lyer pths (i.e., ommon pool) to rry the lient spre pity is reommene to llevite reunnt protetion n remin ost-effetive. In SH-over-WM networks, where WM reovery shemes re only vilble in prts of the network (evolutionry senrio): The isvntge of leving the protetion ompletely in the SH lyer is tht it is iffiult to exploit the full potentils of optil networking The isvntge of shifting the protetion ompletely to the WM lyer (without ny SH protetion in WM prts) is tht the SH pths re protete in segmente wy SH protetion over WM protetion offers the highest egree of relibility, but osteffetive implementtion my only be hieve if working n spre SH pity re trete ifferently in the WM lyer (protetion seletivity or better ommon pool). AKNOWLGMNT This work hs been supporte by the uropen ommission through the ATS projet A205/PANL n by the Flemish Government through IWT projet ITA/950214/INT. RFRNS [1] T.-H. Wu, merging Tehnologies for Fiber Network Survivbility, I ommun. Mg., vol. 33, no. 2, 1995, pp [2] ANSI T1, A Tehnil Report on Network Survivbility Performne, rep. no. 24, Ot [3] K. R. Krishnn, R.. overspike n.. Pk, Improve Survivbility with multi-lyer ynmi routing, I ommun. Mg., July 1995, pp [4]. Johnson, Survivbility Strtegies for robn Networks, Pro. I GLOOM 96, Lonon, U.K., [5] L. Neerlof et l., n-to-en Survivble robn Networks, I ommun. Mg., vol. 33, no. 9, 1995, pp [6] M. Gryseels et l., Optiml esign for Servie Resiliene in ATM on SH kbone Networks, Pro. I ATM 98 Wksp., Firfx, VA, My [7] J. Mnhester, Fult etetion n Propgtion in Trnsport Networks, Pro. 1st Int l. Wksp. esign of Relible ommun. Networks, rugge, elgium, [8] A. Autenrieth et l., Simultion n vlution of Multilyer robn Networks, Pro. 1st Int l. Wksp. esign of Relible ommun. Networks, rugge, elgium, [9] A. MGuire n P. onenfnt, Stnrs: The lueprints for Optil Networking, I ommun. Mg. vol. 36, 1998, pp [10] M. Gryseels et l., ommon Pool Survivbility for ATM over SH Ring Networks, Pro. 8th Int l. Symp. Network Plnning, Sorrento, Itly, Ot [11] O. Gerstel, Opportunities for Optil Protetion n Restortion, Pro. OF 98, Sn Jose, A, IOGRAPHIS PIT MSTR (piet.emeester@inte.rug..be) reeive his Ph egree from the University of Gent t the eprtment of Informtion Tehnology (INT) in He worke for the Interuniversity Miro letronis enter (IM) n beme professor t the University of Gent in 1993 where he is urrently tehing teleommunition networks n where he is responsible for the reserh on ommunition networks. He is senior member of I n he ws or is member of severl tehnil progrm ommittees. He I ommunitions Mgzine August

7 ws involve in bout 15 uropen SPRIT, RA n ATS projets. He ws o-eitor of the speil issue of the I ommunitions Mgzine: Optil Networks Reserh in urope, He ws hirmn of the First Interntionl Workshop on the esign of Relible ommunition Networks in 1998 (RN98) n he is member of the eitoril bor of the journls: OPTIAL NTWORKS MAGAZIN n JOURNAL PHOTONI NTWORK OMMUNIATIONS. He publishe over 300 publitions in the fiel of optoeletronis n brobn networks. His urrent interests re relte to brobn ommunition networks (IP, ATM, SH, WM, ess) n inlue network plnning, network n servie mngement, teleom softwre, internetworking, et. MIHAL GRYSLS (mihel.gryseels@inte.rug..be) reeive n M.S. egree in eletril engineering in 1996 from the University of Gent, elgium. Sine 1996 he hs been working s reserher for the Fun for Sientifi Reserh of Flners (F.W.O.) t the eprtment of Informtion Tehnology (INT), University of Gent, where he is stuying multilyer networks. urrently, he is visiting reserher with Telstr orportion, Austrli. His min reserh interests inlue moeling, plnning, n evlution of multitehnology network rhitetures. AHIM AUTNRITH (him@lkn.e-tehnik.tu-muenhen.e) reeive his ipl.-ing. egree t the Munih University of Tehnology, Germny, in Sine then he hs been member of the reserh stff t the Institute of ommunition Networks t the Munih University of Tehnology, where he ws responsible for the ATS Projet PANL from 1996 to He is urrently working on his Ph.. in the re of multitehnology resiliene, inluing IP. ARLO RIANZA (rlo.brinz@itltel.it) reeive his r. ng. egree in eletronis t Politenio i Milno n Mster s in informtion tehnology t the efriel reserh institute in Miln in He joine Itltel S.p.A. entrl R& in 1994 n ws responsible for Itltel prtiiption in both the PANL n MISA ATS projets. At the en of 1998 he joine the Opertions n Support Systems ivision of INFOSTRAA S.p.A. His working experiene enompsses ll TMN lyers. LAURA ASTAGNA (L.stgn@sirti.it) reeive otorte egree in eletroni engineering from Politenio i Milno in She joine the Sirti R& ept. in 1996 s stger to stuy new methoologies to pln n ATM multiservie network. Sine 1997 she hs worke in the R& eprtment of Sirti;she is engge in plnning ATM, SH, n WM networks, in interworking between IP n WM, n s elegte to ATM-Forum n ITU-T SG 12/13/15. GIULIO SIGNORLLI (G.Signorelli@sirti.it) grute in eletroni engineering, speiliztion teleommunitions, from University of Pvi, Itly, in He worke in the eletrooptis lbortories of this university. In 1995 he joine the R& ivision of Sirti. He ws involve in the trnsport network re eling with SH n WM network plnning. He prtiipte in Teleom Itli SH plnning projets n severl turnkey network projets for other opertors. He is member of ITU n TSI stuy groups involve in optil networking. He hs reently jointe Infostr, the new Itlin telo opertor, where he is involve in the SH n WM network strtup phse. RORTO LMNT (roberto.lemente@selt.it) reeive his egree in eletroni engineering from Politenio of Torino in uring the next yer he joine SLT, the orporte teleommunition reserh enter of Teleom Itli, n strte working in the Network Plnning n Mngement ivision. His interests re in the imensioning of trnsport SH/WM networks, ressing both the long istne n regionl network shres. He ws involve in severl URSOM n ATS projets n hs been o-uthor of ppers regring trnsport network plnning n single- n multilyer reovery shemes in severl onferenes for the lst three yers. MAURO RAVRA (muro.rver@selt.it) reeive egree in eletroni engineering from the Politenio of Torino in 1988; uring the sme yer he joine SLT, the orporte teleommunitions reserh enter of the Teleom Itli group. Sine 1992 he hs worke in the Network Plnning n Mngement eprtment, where he hs been responsible for projet whih ontribute to the funmentl plns of the ess n trnsport network of Teleom Itli. ANRZJ JAJSZZYK [F 99] (Anrzej.Jjszzyk@itti.om.pl) reeive his M.S. egree in eletril engineering in 1974, n his Ph.. egree in ommunitions in He is professor t the University of Tehnology n Agriulture, ygoszz, Poln. His reserh interests inlue high-spee networking s well s network n servie mngement. He hs publishe over 150 ppers n six books, n hols 19 ptents. He ws n eitor of I Trnstions on ommunitions n founing eitor of I Globl ommunitions Newsletter. He is itor-in-hief of I ommunitions Mgzine. He hs orgnize severl symposi, n serve on numerous progrm n orgnizing ommittees. ARIUSZ JANUKOWIZ (jnukow@unitel.om.pl) finishe his Mster s stuies t the University of Tehnologies in He worke on mngement of ommunition networks t the Institute of ommunition n Informtion Tehnologies. He ws involve in the PANL projet in the mngement-relte prt. Sine 1999 he is involve in the evelopment of Internet ess networks n swithing mngement t the Teleommunition Stuies n esign ureu UNITL Lt., Poznn, Poln. KRISTOF VAN OORSLAR (kristof.vn.oorselere@is.belgom.be) grute s n engineer from the University of Gent in In tht yer he joine the reserh group of Prof. Piet emeester t the University of Gent n stuie there the fult mngement n survivbility in WM/SH/ATM trnsport networks. In 1998 he left for elgom n is working there on the long-term lese line rhiteture. YOHNOSUK HARAA (hr@netsrg.tnl.ntt.o.jp) grute from the Kyoto University Fulty of ngineering, n his mjor is on utomti ontrol. After finishing his Mster s egree, he entere NTT letril ommunition Lbortories. He hs been working on teletrffi stuy, network plnning lgorithms, network plnning tools, ynmi routing lgorithms, evelopment of network opertion systems, seurity rhiteture, ATM stnriztion, n overses ATM pplition trils with AT&T, T, n K. He is urrently prtiipting in multimei pplitions evelopment in soil siene. 76 I ommunitions Mgzine August 1999